Support BART models for classification

Hello again,

I'm trying to convert this adaptation of Bart Large MNLI: https://huggingface.co/joeddav/bart-large-mnli-yahoo-answers

It returns the following error (but the base Bart Large MNLI model works well):

Traceback (most recent call last):
  File "/home/ubuntu/.local/bin/./ct2-transformers-converter", line 8, in <module>
    sys.exit(main())
  File "/home/ubuntu/.local/lib/python3.10/site-packages/ctranslate2/converters/transformers.py", line 445, in main
    converter.convert_from_args(args)
  File "/home/ubuntu/.local/lib/python3.10/site-packages/ctranslate2/converters/converter.py", line 50, in convert_from_args
    return self.convert(
  File "/home/ubuntu/.local/lib/python3.10/site-packages/ctranslate2/converters/converter.py", line 89, in convert
    model_spec = self._load()
  File "/home/ubuntu/.local/lib/python3.10/site-packages/ctranslate2/converters/transformers.py", line 62, in _load
    return loader(self._model_name_or_path)
  File "/home/ubuntu/.local/lib/python3.10/site-packages/ctranslate2/converters/transformers.py", line 85, in __call__
    spec = self.get_model_spec(model)
  File "/home/ubuntu/.local/lib/python3.10/site-packages/ctranslate2/converters/transformers.py", line 146, in get_model_spec
    pre_norm=model.config.normalize_before,
  File "/home/ubuntu/.local/lib/python3.10/site-packages/transformers/configuration_utils.py", line 257, in __getattribute__
    return super().__getattribute__(key)
AttributeError: 'BartConfig' object has no attribute 'normalize_before'

Thanks in advance!

Start with ctranslate2/converters/transformers.py and the ct2-transformers-converter entry point shown in the traceback. Reproduce the conversion for joeddav/bart-large-mnli-yahoo-answers and compare it with the working base BART Large MNLI model, focusing on the missing BartConfig.normalize_before attribute. Done means the adapted BART model converts without this error.